Gas turbine blade

ABSTRACT

A blade for a gas turbine includes an internally formed passage (a) for cooling the blade with steam. The ratio (t/c) of blade thickness (t) to chord length (c) should fall within the range of 0.10 to 0.15.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

The present invention relates to a blade for a gas turbine, and moreparticularly to a gas turbine blade adapted to be internally cooled by acoolant steam flowing through the interior of the gas turbine blade.

2. Description of the Related Art

A conventional gas turbine for a combined cycle is shown in FIG. 5 toFIG. 7. As is shown in FIG. 5, the gas turbine 1 includes a plurality ofalternately disposed stationary blades 2 and moving blades 4. A main gasflow of high temperature is fed to the gas turbine 1 from a combustor(not shown) to effect the moving blades 4 via the stationary blades 2and rotate a rotor of the turbine at a high speed to thereby generatepower.

Each of the stationary blades 2 of the turbine is provided with coolantsteam pipes 3. Further, a plurality of radial coolant steam flow pathsections a are formed inside the stationary blade 2, as can be seen inFIG. 7, wherein these coolant steam flow path sections are seriallyinterconnected at a base end portion and a tip end portion of thestationary blade to thereby form a serpentine passage that extends froma leading edge side to a trailing edge side of the turbine stationaryblade.

When the gas turbine is put into operation with the high temperaturemain gas being fed to the gas turbine, the stationary blade 2 is heated.In this case, however, the coolant steam is supplied to the coolantsteam flow path section a formed inside of the stationary blade 2 of theturbine and located adjacent to the leading edge thereof to sequentiallyflow through the coolant steam flow path sections which form theserpentine passage. Hence, the stationary blade 2 of the turbine can becooled from the internal surfaces thereof. Moreover, the coolant steamused for cooling the stationary blade 2 of the turbine is subsequentlyrecovered through the coolant steam pipe 3 to be supplied to asucceeding system.

As can be seen from the above, in the combined cycle plant where the gasturbine is used in combination with a steam turbine, steam cooling hasbeen proposed for cooling the gas turbine to enhance the thermalefficiency.

Next, a conventional air impingement cooling scheme is illustrated inFIG. 8 to FIG. 10. Inserts 5 a, 5 b and 5 c are respectively disposedwithin radially partitioned compartments of a stationary blade 2 for aturbine, each insert having a predetermined gap relative to the innerwall surface of the compartment and a number of small holes 6 are formedtherein. Additionally, a number of small holes 7 are also formed in thesurface of the stationary blade 2 of the turbine in a predeterminedpattern.

In the turbine stationary blade 2 of the structure mentioned above, thecooling air flowing into the inserts 5 a, 5 b and 5 c is ejected throughthe individual small holes 6 formed in the inserts 5 a, 5 b and 5 c toimpinge on the inner wall surfaces of the stationary blade 2 of theturbine to thereby cool the stationary blade 2 from the inside (refer toFIG. 10). Subsequently, the cooling air is ejected from the small holes7 formed in the surface of the stationary blade to film cool the latter.

When impingement cooling using the cooling air, the ratio t/c of bladethickness t to chord length c of the stationary blade 2 of the turbine,as shown in FIG. 7, is ordinarily selected so as to be at least 0.2 inorder to facilitate insertion of the inserts 5 a-5 c into the interiorof the stationary blade 2 of the turbine and ensure an uniform pressuredistribution within the interiors of the individual inserts.

Nevertheless, a turbine stationary blade of a profile having the bladethickness/chord length ratio t/c mentioned above requires a large amountof coolant steam, e.g., a major proportion of the steam for the steamturbine of the combined cycle, because convection cooling must beadopted for steam cooling.

Furthermore, leakage of the coolant steam in the coolant steam pipingwill seriously affect the combined cycle (bottoming cycle), and possiblymake the plant nonfunctional.

Also, the diameters of the pipes used in a complicated coolant steampiping system have to be increased due to the demand for a large amountof coolant steam, and this presents problems with respect to the costand the space for installation of the coolant steam piping arrangementof individual pipes.

OBJECT OF THE INVENTION

In light of the state of the art described above, it is an object of thepresent invention to provide a blade for a combined cycle gas turbinethat can be effectively cooled with a proper amount of steam, to therebysolve the problems mentioned above.

SUMMARY OF THE INVENTION

The present invention provides the following means for achieving theabove object.

That is, a blade for a gas turbine according to the present inventionincludes an internally formed coolant steam passage for cooling theblade with coolant steam flowing through the coolant steam passage, andis characterized in that the ratio of blade thickness of the gas turbineblade to chord length thereof is not greater than 0.15.

By virtue of the structure of the gas turbine blade mentioned above, theflow velocity of the coolant steam flowing through the coolant steampassage increases as the ratio of the blade thickness to the chordlength becomes smaller for a given constant flow rate of the coolantsteam. Further, the heat transfer rate at the inner wall surface portionof the blade increases correspondingly.

Accordingly, by selecting the ratio t/c of the blade thickness to thechord length so as to be not greater than 0.15, which corresponds to aminimum heat transfer rate required in view of the required coolingperformance of the turbine blade, not only can the turbine blade becooled effectively, but also an appropriate steam flow rate for thecombined cycle can also be maintained.

Thus, even when leakage of the coolant steam occurs in the coolant steampiping, the effects of such leakage on the plant's performance can besuppressed to a minimum because the inherent amount of coolant steam issmall. Furthermore, due to the small amount of coolant steam required,the steam piping system can be implemented using pipes of smalldiameters. Thus, the cost and the space required for installation of thecomplicated piping system for the plant can be reduced.

Furthermore, it is preferred to implement the blade for the gas turbineaccording to the present invention such that the ratio of the bladethickness of the gas turbine blade to the chord length thereof fallswithin a range of 0.10 to 0.15.

Owing to the structure mentioned above, effective cooling of the turbineblade with the coolant steam is performed while ensuring effectivereutilization of the recovered coolant steam.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view showing a stationary blade for a turbineaccording to an exemplary embodiment of the present invention.

FIG. 2 is a graph illustrating the relationship between the bladethickness/chord length ratio of the stationary blade of the turbineaccording to the exemplary embodiment shown in FIG. 1 and steam flowvelocity.

FIG. 3 is a graph illustrating the relationship between the bladethickness/chord length ratio and heat transfer rate at an inner wallsurface portion on one hand, and metal temperature on the other hand, ofthe stationary blade for the gas turbine according to the exemplaryembodiment shown in FIG. 1.

FIG. 4 is a graph illustrating the relationship between the bladethickness/chord length ratio and loss of steam pressure in thestationary blade for the turbine according to the exemplary embodimentshown in FIG. 1.

FIG. 5 is a schematic view of a structure of a conventional steamcooling type gas turbine.

FIG. 6 is a detail view showing a stationary blade for the conventionalsteam cooling type turbine.

FIG. 7 is a view taken along the direction indicated by the arrows A inFIG. 6 and shows a cross section of a conventional stationary blade fora steam cooling type turbine.

FIG. 8 is a vertical section of a conventional stationary blade for anair cooling type turbine.

FIG. 9 is a cross-sectional view showing a conventional stationary bladefor an air cooling type turbine.

FIG. 10 is a detail view showing the portion B shown in FIG. 9 forillustrating cooling of an inner surface of a stationary blade for aturbine using cooling air.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The present invention will be described in detail in conjunction withwhat are presently considered preferred embodiments thereof withreference to FIG. 1 to FIG. 4 of the accompanying drawings. Moreover,the parts or portions mentioned previously in conjunction with therelated art are denoted by the same reference numerals, and so repeatdescription thereof is omitted. Accordingly, the description whichfollows will mainly be directed to the present invention.

In the following description, like reference numerals designate like orcorresponding parts throughout the drawings. Also in the followingdescription, it is to be understood that terms such as “right”, “left”,“top”, “bottom” and the like are words of convenience and are not to beconstrued as limiting terms.

FIG. 1 is a sectional view showing a stationary blade for a turbineaccording to an exemplary embodiment of the present invention. Referringto the figure, a stationary blade 2 for a steam cooling type gas turbinehas a blade profile such that the ratio t/c of blade thickness t tochord length c is not greater than 0.15. Preferably, the ratio t/c ofthe blade thickness to the chord length should be selected so as to liewithin a range of 0.10 to 0.15.

In the stationary blade 2 of the turbine structured as mentioned above,the coolant steam can enter a coolant steam flow path section a providedinside of the stationary blade 2 of the turbine on the leading edge sidethereof and flow in the radial direction from a base end portion of thestationary blade of the turbine toward the tip end thereof.Subsequently, the coolant steam enters a succeeding coolant steam flowpath section at the tip end of the blade to reverse flow from the tipend side toward the base end portion. In a similar manner, the coolantsteam successively flows from the leading edge side to the trailing edgeside through the coolant steam flow path sections formed inside of thestationary blade 2, to thereby cool the turbine stationary blade 2 fromthe interior.

In this case, it is noted that when the flow rate of the coolant steamis constant, the flow velocity of the coolant steam increases as theratio t/c of the blade thickness t to the chord length c becomessmaller, as can be seen from the relationship between the bladethickness/chord length ratio t/c of the turbine stationary blade 2 andthe steam flow velocity illustrated in FIG. 2.

Furthermore, as is apparent from the relationship between the bladethickness/chord length tic and heat transfer rate at an inner wallsurface portion of the blade on one hand and the metal temperature onthe other hand, shown in FIG. 3, the heat transfer rate at the innerwall surface portion increases as the ratio t/c of the blade thickness tto the chord length c of the turbine stationary blade 2 becomes smaller.

Here, it is noted that a minimum heat transfer rate α₁ required in viewof the cooling performance of the turbine blade may differ depending onthe external gas temperature conditions and pressure condition of thecooled blade. Nevertheless, the minimum heat transfer rate may bedefined as the internal heat transfer rate required so that the metaltemperature of the blade will not exceed a designed reference value(e.g. not higher than 900° C.). As can be seen in FIG. 3, when the ratiot/c of the blade thickness t to the chord length c exceeds a value of0.15, the heat transfer rate becomes excessively low, and as a result,the metal temperature of the blade becomes excessively high and exceedsthe designed reference value.

Accordingly, by selecting the ratio t/c of the blade thickness t to thechord length c of the stationary blade 2 for the gas turbine to be notgreater than 0.15, which corresponds to the minimum heat transfer rateα₁ required in view of the cooling performance of the turbine blade, notonly can the turbine blade be cooled effectively, but a low steam flowrate appropriate for the combined cycle can be maintained.

Thus, even when leakage of the coolant steam occurs in the coolant steampiping, effects of such leakage on the plant's performance can besuppressed to a minimum because the inherent amount of coolant steam issmall. Further, owing to the small amount of the coolant steam required,the steam piping system can be implemented using pipes of smalldiameter. Thus, the cost and space for the complicated piping system ofthe plant can be reduced.

Further, FIG. 4 is a graph of the relationship between the ratio t/c ofthe blade thickness t to the chord length c and steam pressure loss. Ascan be seen from this figure, when the ratio t/c of the blade thicknesst to the chord length c is smaller than 0.10, pressure loss of the steamincreases excessively, rendering the blade inappropriate for steamcooling. Accordingly, it is preferred that the ratio tic of the bladethickness t to the chord length c lies within a range of 0.10 to 0.15 inorder to obtain effective cooling of the turbine blade and effectivereutilization of the recovered coolant steam.

As is apparent from the foregoing, the turbine blade according to thepresent invention can exhibit the cooling performance required of theturbine blade at a low steam flow rate because the ratio of the bladethickness to the chord length is not greater than 0.15. Further, bymaking the ratio of the blade thickness to the chord length in the rangeof from 0.10 to 0.15, in addition to the above-mentioned advantageouseffect, effective reutilization of the recovered coolant steam can berealized.

In the foregoing, the embodiment of the present invention which isconsidered preferable at present and other alternative embodiments havebeen described in detail with reference to the drawings. It should,however, be noted that the present invention is never restricted tothese embodiments but other various applications and modifications ofthe blade for the gas turbine can easily be conceived and realized bythose skilled in the art without departing from the spirit and scope ofthe present invention.

What is claimed is:
 1. A blade for a gas turbine equipped with aninternally formed coolant steam passage for cooling the blade withcoolant steam flowing through said coolant steam passage, wherein aratio of blade thickness of said gas turbine blade to chord lengththereof lies within a range of 0.10 to 0.15.